Steam expandable polymeric composition and method

ABSTRACT

A steam expandable alkenyl aromatic polymer composition and methods of preparation are provided. The composition is mixed with a volatile blowing agent containing, as a primary blowing agent, dichlorodifluoromethane and as a secondary blowing agent, a halogenated hydrocarbon, a hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms, the secondary blowing agent having a normal atmospherpic boiling point of from about 0° to about 100° C. The composition may be expanded by steam immediately after extrusion foaming or may be expanded after aging of the composition which has been impregnated with volatile blowing agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 672,001, filed Nov. 16, 1984.

BACKGROUND OF THE INVENTION

This invention relates to a steam expandable polymeric composition and method of preparation, and more particularly to a steam expandable aklenyl aromatic polymer composition with mixed volatile blowing agent and methods of expanding the composition in atmospheric steam.

Steam expansion is a widely applied technology in preparing a styrenic polymer foam. Atmospheric steam (i.e., steam at atmospheric pressure) is often used to expand expandable polystyrene (EPS) resins. Both on-line and off-line steam expansion techniques have been utilized.

On-line steam expansion occurs immediately following extrusion of expanded polystyrene foam. For example, Japan Kokai published application No. 42-24071 discloses that steam may be used to expand polystyrene foam at the die of a foam extrusion line. Off-line steam expansion utilizes steam to expand polystyrene resin particles which have been impregnated with a volatile blowing agent. For example, both Kajimura, U.S. Pat. No. 3,994,840, and Yamai, U.S. Pat. No. 4,065,411, teach impregnation of polystyrene particles with a volatile blowing agent such as n-pentane, aging the expandable particles, and then expanding the particles to a cellular foamed structure by exposure to steam.

However, most commonly used blowing or expanding agents have a deficiency either in the rate of expansion of the polymer, the degree of expansion of the polymer, or the final properties of the expanded polymeric foam. For example, a polystyrene foam expanded with a mixture of dichlorodifluoromethane (FC-12) and methyl chloride expands too slowly and to too low a degree in steam to be practical for commercial activity. A polystyrene foam made with pure trichloromonofluoromethane or pentane expands rapidly in steam but has several deficiencies in expansion characteristics, results in a softening of the final foam structure, and prevents expansion of the foam to very low densities.

Accordingly, the need exists in the art for a steam expandable alkenyl aromatic polymer composition and method for preparation which provides rapid expansion of the composition in steam to very low densities without adversely affecting the final properties or structure of the foam.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a steam expandable polymeric composition and process which expands rapidly to very low densities. According to one aspect of the invention, the expandable composition comprises an alkenyl aromatic polymer having a glass transition temperature between about 60° to about 100° C. together with a volatile blowing agent. The volatile blowing agent comprises a mixture of from about 40 to about 90% by weight of a primary blowing agent comprising dichlorodifluoromethane (FC-12) and from about 10 to about 60% by weight of a secondary blowing agent comprising a halogenated hydrocarbon, a hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms, the halogenated hydrocarbon, hydrocarbon, or aliphatic alcohol having a normal atmospheric boiling point of from about 0° to about 100° C. Optionally, small amounts of a silane crosslinking agent may also be added.

It has been found that this combination of alkenyl aromatic polymer and mixed blowing agent provides a rapidly expanding composition in atmospheric steam to low densities [i.e., below about 2.0 pcf (32.0 Kg/m³)]. In addition to having a glass transition temperature (Tg) between about 60° to about 100° C., the polymer preferably has a weight average molecular weight of equal to or less than about 250,000. Such molecular weight may be determined by gel permeation chromatography (GPC), for example. The blowing agent is selected to have adequate solubility to plasticize the polymer. Thus, the unique plasticizing effect of the secondary blowing agent also helps lower the glass transition temperature of the polymer thereby facilitating steam expansion. The secondary blowing agent then diffuses out of the composition without any adverse effect on the final foam properties. This is particularly advantageous since other plasticizing agents such as solid or liquid compounds mixed into the polymer will permanently remain in the final cellular structure.

The expandable composition of the present invention may be used to produce a very low density moldable material by off-line expansion and also produce a lower density extruded foam by on-line application of steam. To produce the very low density moldable material, the volatile mixed blowing agent of the present invention is impregnated into a granular alkenyl aromatic polymer having a glass transition temperature between about 60° to about 100° C. The polymer impregnated with blowing agent may then be heated to activate the blowing agent and expand the composition. Steam may be used to provide the heat for this initial expansion, or alternatively, the expanded composition may be subjected to further single or multiple expansions in the presence of atmospheric steam.

A low density extruded foam may be produced in accordance with the present invention by melt processing under pressure an alkenyl aromatic polymer having a glass transition temperature between about 60° to about 100° C. with the mixed volatile blowing agent of the present invention to form a flowable admixture. The admixture is then extruded through a die to a zone of lower pressure where the blowing agent is activated to expand the composition to a cellular structure. This cellular foam is then exposed to atmospheric steam to cause further expansion of the foam to even lower densities. The exposure of the admixture to lower pressure and to atmosphere steam may occur substantially simultaneously.

Accordingly, it is an object of the present invention to provide a steam expandable polymeric composition and method of preparation which provides rapid expansion of the composition in steam to very low densities and without adverse affect on the final properties of the foam structure. This and other objects and advantages of the invention will become apparent from the following detailed description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a steam chamber for the on-line expansion of polymeric foam;

FIG. 2 is a graph comparing the expansion ratio versus expansion time for polystyrene foams having different blowing agents and/or levels of silane crosslinking agent based on data set forth in Example I;

FIG. 3 is a graph comparing the reduction in foam density versus exposure time for the on-line steam expansion of a polystyrene foam with different blowing agents based on data set forth in Example IV; and

FIG. 4 is also a graph comparing the reduction in foam density versus exposure time for the on-line steam expansion of a polystyrene foam with different blowing agents based on data set forth in Example IV.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The expandable polymeric composition of the present invention comprises an alkenyl aromatic polymer resin having a glass transition temperature of between about 60° to about 100° C. and having a weight average molecular weight equal to or less than about 250,000.

By the term "alkenyl aromatic polymer" resin, is meant a solid polymer of one or more polymerizable alkenyl aromatic compounds and, optionally, at least one copolymerizable monomer. The polymer or copolymer comprises, in polymerized form, at least 70% by weight of at least one alkenyl aromatic monomer having the general formula: ##STR1## where Ar represents an aromatic hydrocarbon radical, or an aromatic halo-hydrocarbon radical of the benzene series, and R is hydrogen, a methyl group, or an ethyl group. Examples of such alkenyl aromatic polymers are the solid homopolymers of styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, or ar-bromostyrene, the solid copolymers of two or more of such alkenyl aromatic compounds with minor amounts of other readily polymerizable olefinic compounds such as acrylonitrile, anhydrides of maleic or itaconic acids, or rubber-reinforced (either synthetic or natural) styrene polymers, and the like.

The blowing agent of the present invention comprises a mixture of a primary and a secondary blowing agent. The primary blowing agent comprises dichlorodifluoromethane in an amount of from about 40 to about 90% by weight, and preferably about 60 to about 80% by weight, of total blowing agent. The secondary blowing agent comprises a halogenated hydrocarbon, a hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms, wherein the secondary blowing agent has a normal atmospheric boiling point of from about 0° to about 100° C., in an amount of from about 10 to about 60% by weight, and preferably about 20 to about 40% by weight, of total blowing agent.

It has been found that this combination of primary and secondary blowing agents in the weight percentages given above provide, in combination with the alkenyl aromatic polymer of the present invention, a composition which is readily and rapidly expandable in atmospheric steam to very low densities below about 0.6 pcf (9.6 Kg/m³). The unique plasticizing effect of the secondary blowing agents aid in lowering the glass transition temperature of the polymer rendering it readily expandable in atmospheric steam. After expansion, the secondary blowing agent diffuses out of the cellular structure. However, the use of too much secondary blowing agent, above the percentage range disclosed above, may cause too much plasticizing effect on the composition resulting in a permanent softening of the final cellular structure and an inability to expand the composition to very low densities.

Likewise, the weight percentage of primary blowing agent in the composition is also important. Too much of the primary blowing agent results in a composition which does not rapidly expand in steam. If the level of primary blowing agent is too low, the foam may collapse, may fail to recover its volume after expansion, or may have too low a heat distortion temperature. The preferred concentration of total blowing agent to alkenyl aromatic polymer is from about 10 to about 30 parts per hundred parts by weight of polymer.

It has been found that the optional addition of about 0.01 to about 2.0 parts per hundred of an azido functional silane as a crosslinking agent to the overall composition further improves the steam expandability of the composition of the present invention. Suitable azido functional silane compounds include the group of azido trialkoxysilanes such as 2-(trimethoxysilyl)ethyl phenyl sulfonyl azide (commercially available from Petrarch Systems, Inc., Bristol, Pa.) and (triethoxysilyl)hexyl sulfonyl azide (commercially available as Azcup D-98 from Hercules, Inc., Wilmington, Del.).

The expandable composition of the present invention may be used to produce very low density material by means of either on-line or off-line expansion techniques. A very low density moldable material may be produced by impregnating the volatile mixed blowing agent of the present invention into a granular alkenyl aromatic polymer composition having a glass transition temperature between about 60° to about 100° C. The polymer impregnated with blowing agent is activated by heating causing the composition to expand. Typically, the expansion produces foam beads having a density of about 2-3 pounds per cubic foot (pcf) (32.0-48.1 Kg/m³). After aging, preferably for a period of from about 1 to about 7 days, the foam beads are exposed to steam at substantially atmospheric pressure for a period of from several seconds to two hours, and preferably about 5 to about 15 minutes. Both single and multiple steam expansions are contemplated. This causes further expansion of the beads to densities as low as 0.2 pcf (3.2 Kg/m³). The very low density foam beads may then be molded in accordance with known techniques.

An alternative method in accordance with the present invention is to produce a low density extruded foam by exposing the foam to steam after the foam leaves the extruder. This exposure to steam may take place after the extruded foam has been aged for a period of time or may take place immediately after the foam has left the die of an extruder.

FIG. 1 illustrates, in a schematic diagram, such an on-line steam expansion technique. The mixed volatile blowing agent of the present invention is melt processed under pressure with an alkenyl aromatic polymer having a glass transition temperature between about 60° to about 100° C. to form a flowable admixture or polymer gel 10. Polymer gel 10 then exits extruder 12 through die 14 to zone of lower pressure. In this instance, the zone of lower pressure is a steam chamber 20 having an inlet 22 for the addition of steam to the chamber at substantially atmospheric pressure. Steam enters the chamber through a porous distributor 23.

Steam chamber 20 has walls 24, 25 which may be of any suitable material including metal, Plexiglass®, or a polymer resin such as a polycarbonate. Outer wall 25 acts as a shield for inner wall 24. The end of steam chamber 20 is sealed with a flexible rubber seal 28. As the polymer gel 10 exits die 14 it initially expands to a cellular structure having a density typically between 2 to 3 pcf (32.0 to 48.1 Kg/m³). Exposure of the foam to atmospheric steam in steam chamber 20 for a time causes further expansion to lower densities. A pair of driven rolls 30, 32 pull the cellular foam out of the steam chamber.

Preferably, the steam chamber is designed to provide sufficient residence time for the steam to act on and expand the extruded foam. Typically, such residence time will be a period of from several seconds to several minutes. Accordingly, it can be seen that rapid expansion in steam is quite important in such an on-line technique. The mixed blowing agent of the present invention provides that rapid expansion capability.

The very low density foams produced by the above on- and off-line methods has several benefits including low final product cost because of the very low density product which is produced and increased flexibility of the foam structure because of the thinness of the cell walls. The foams produced may find use as insulation and cushion packaging.

The specific working examples that follow are intended to illustrate the invention but are not to be taken as limiting the scope thereof. In the examples, parts and percentages are by weight unless otherwise specified or required by the context.

EXAMPLE I

The apparatus used in this and other examples was a 11/4" screw type extruder having two additional zones for mixing and cooling at the end of usual sequential zones for feeding, melting, and metering. An opening for blowing agent injection was provided on the extruder barrel between the metering and mixing zones. A small styringe-type pump was connected to the blowing agent stream for optional injection of crosslinking agent. At the end of the cooling zone there was attached a die orifice having an opening of rectangular shape. The height of the opening, called die gap hereinafter, was adjustable while its width was fixed at 0.25 inches (6.35 millimeters).

A granular polystyrene with a weight average molecular weight of about 200,000 and a glass transition temperature of less than 100° C., was uniformly mixed with a small amount of dibutyl tin dilaurate condensation catalyst (T-12, available from M & T Chemicals, Inc.), barium stearate and talcum powder. The mixture was fed into the extruder at an essentially uniform rate of about 10 pounds per hour. A premixed blowing agent was injected into the extruder at a predetermined rate. Blowing agents used in this example were a 70/30 by weight mixture of dichlorodifluoromethane (FC-12) and trichloromonofluoromethane (FC-11) for the other tests. An azido functional silane, 2-(trimethoxysily ethylphenylsulfonyl)azide (C-T2905 distributed by Petrarch Systems, Inc.), was injected to the blowing agent stream at a uniform predetermined rate. The extruder zones were maintained at 170°, 190° and 200° C. for feeding, melting, metering, and mixing zone, respectively. The temperature of the cooling zone was adjusted so that the gel could be cooled down to a uniform temperature for optimum foam expansion.

At a fixed die opening for a given blowing agent system where all formulations provided good quality foams, both pressures at the die and extruder discharge were recorded and samples were taken. The foam samples were aged for about one month at ambient temperature prior to property determination and secondary foaming tests.

Secondary foaming was conducted both by atmospheric steam and by hot air (not an example of the present invention). Foam slugs of about 0.25 inch (6.35 millimeters) thickness were sliced out of foam strands and aged for about one day at ambient temperature prior to secondary expansions. After exposure to atmospheric steam for varying lengths of time, foam specimens were aged at ambient temperature while their weights and volumes were monitored. Highly expanded foam specimens shrank when taken out of steam but recovered to the final steady state volumes in about two days. Expansion tests in hot air were conducted similarly with the exception that expansion or shrinkage of a foam specimen in the oven was permanent, not needing ambient aging for volume recovery.

Tables IA and IB show the good results utilizing of the mixed FC-12/FC-11 blowing agent of the present invention. The tables also show that incorporation of an azido functional silane gradually raises both the die pressure and the extruder discharge pressure. Other noticable effects of silane include enlargement in cell size and cross sectional size which are desirable in foam extrusion.

The remarkable effects of the mixed blowing agent of the present invention and silane crosslinking agent on steam expandability of extruded polystyrene foam is seen in Table IB and the graph of FIG. 2, which represents the data reported in Table IB. The optional addition of silane enhances both the rate and extent of foam expansion in steam. At a silane level of 0.2 pph, the foam expands about twice as much as Test No. 1 to an expansion ratio exceeding 280 in two hours. At this long exposure to steam, Tests No. 1 and 2, one with a low level (0.05 pph) of silane, suffered thermal collapse; the expansion ratios are lower than those at one hour exposure.

The type of blowing agent has a significant effect on steam expandability of the extruded polystyrene foams. The FC-12/FC-11 mixed blowing agent of the present invention showed good expansion ratios. FC-11, having a high solubility in polystyrene, effectively plasticizes polystyrene for faster and greater expansion.

The effect of crosslinking is also seen in hot air expansion tests. As Table IC shows, foams lightly crosslinked with an azido functional silane expand more and withstand hot air longer than the control. For example, a foam crosslinked with 0.15 pph or higher level of azido silane keeps expanding during 90 minutes exposure to 100° C. air while the control collapses within 60 minutes at the same temperature. Although less pronounced, the crosslinking effect persists at a lower level of silane as well.

EXAMPLE II

The apparatus and its operating procedure used for tests in this example are the same as in Example 1. The polymer feedstock employed for tests in this example was a 50/50 by weight blend of polystyrene with Mw 200,000 and polystyrene with Mw 300,000. The mixture of polystyrenes had a glass transition temperature of less than 100° C. Approximately 0.05 pph T-12, 0.1 pph barium stearate and 0.1 pph talcum powder were mixed in the granular blend of two polystyrenes and fed into the extruder at a uniform rate of 10 pounds per hour.

                                      TABLE IA                                     __________________________________________________________________________                                Azido                                                                              Foaming     Ex-                                 Blowing Agent  BaSt                                                                               Talc                                                                               T-12                                                                               Silane                                                                             Gel  Die                                                                               Die truder                                                                             Foam                                                                               Foam                                                                               Foam Cell               Test                                                                              Type                                                                               Ratio                                                                              Level                                                                              Level                                                                              Level                                                                              Level                                                                              Level                                                                              Temp.                                                                               Gap                                                                               Press.                                                                             Press.                                                                             Thick.                                                                             Width                                                                              Density                                                                             Size               No.                                                                               (1) (2) (3) (4) (5) (6) (7) (8)  (9)                                                                               (10)                                                                               (11)                                                                               (12)                                                                               (13)                                                                               (14) (15)               __________________________________________________________________________     1  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              --  135  0.914                                                                             46.4                                                                               77.3                                                                               17.0                                                                               25.7                                                                               34.0 0.16                  FC-11                                                                       2  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              0.05                                                                                 134.5                                                                             0.914                                                                             39.7                                                                               70.3                                                                               16.5                                                                               27.2                                                                               33.0 0.27                  FC-11                                                                       3  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              0.1 134  0.914                                                                             45.0                                                                               78.0                                                                               22.4                                                                               30.5                                                                               35.2 0.22                  FC-11                                                                       4  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              0.15                                                                               134  0.914                                                                             46.4                                                                               80.9                                                                               22.9                                                                               32.5                                                                               35.9 0.27                  FC-11                                                                       5  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              0.2 134  0.914                                                                             48.5                                                                               84.4                                                                               18.5                                                                               32.8                                                                               36.0 0.27                  FC-11                                                                       6  FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               0.1 0.025                                                                              0.25                                                                               134  0.914                                                                             51.3                                                                               91.4                                                                               24.9                                                                               36.3                                                                               36.5 0.21                  FC-11                                                                       __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of barium stearate mixed in per hundred parts of polymer           (5) = parts of talcum powder mixed in per hundred parts of polymer             (6) = parts of dibutyl tin dilaurate mixed in per hundred parts of polyme      (7) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (8) = temperature of foaming gel in degrees centigrade                         (9) = the gap of die opening in millimeters                                    (10) = the pressure in kilograms per square centimeter at the die              (11) = the pressure in kilograms per square centimeter at the extruder         discharge                                                                      (12) = thickness of foam body in millimeters determined after aging for        about a month                                                                  (13) =  width of foam body in millimeters determined after aging for abou      a month                                                                        (14) = density of foam body in kilograms per square meter determined afte      aging for about a month                                                        (15) = cell size in millimeter in horizontal direction determined per AST      D3576                                                                    

                                      TABLE IB                                     __________________________________________________________________________                       Azido                                                                              Initial                                                  Blowing Agent     Silane                                                                             Expansion                                                                            Expansion Ratio after Expanding in Steam for       Test                                                                              Type   Ratio                                                                              Level                                                                              Level                                                                              Ratio 15 30 1  3  5  7  10 15 20 30 1 2                  No.                                                                               (1)    (2) (3) (4) (5)   sec.                                                                              sec.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              hr.                                                                              hrs.               __________________________________________________________________________     1  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               --  24.6  39.0                                                                              57.6                                                                              81.2                                                                              107                                                                               124                                                                               129                                                                               140                                                                               147                                                                               140                                                                               151                                                                               145                                                                               94                2  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.05                                                                               22.5  66.6                                                                              76.3                                                                              105                                                                               136                                                                               159                                                                               170                                                                               170                                                                               181                                                                               189                                                                               185                                                                               190                                                                              132                3  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.1 24.0  44.2                                                                              66.0                                                                              98.6                                                                              137                                                                               150                                                                               166                                                                               175                                                                               193                                                                               198                                                                               218                                                                               237                                                                              246                4  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.15                                                                               23.7  51.7                                                                              69.7                                                                              102                                                                               138                                                                               154                                                                               168                                                                               175                                                                               187                                                                               197                                                                               218                                                                               252                                                                              218                5  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.2 25.9  57.3                                                                              71.0                                                                              108                                                                               149                                                                               179                                                                               187                                                                               212                                                                               217                                                                               227                                                                               251                                                                               266                                                                              288                6  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.25                                                                               23.4  63.8                                                                              82.1                                                                              109                                                                               136                                                                               162                                                                               173                                                                               155                                                                               183                                                                               186                                                                               186                                                                               161                                                                              172                __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body determined in about two days after          expansion in steam for given period of time                              

                                      TABLE IC                                     __________________________________________________________________________                        Azido                                                                              Initial                                                                              Expansion Ratio after Expanding in                Blowing Agent      Silane                                                                             Expansion                                                                            Hot Air at 100° C. for (min.)              Test                                                                               Type   Ratio                                                                              Level                                                                              Level                                                                              Ratio (6)                                               No. (1)    (2) (3) (4) (5)   3  5  10 15 30 45 60 90                           __________________________________________________________________________     1   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               --  24.6  29.2                                                                              47.8                                                                              68.3                                                                              59.5                                                                              68.8                                                                              54.8                                                                              12.8                                                                              14.0                         2   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.05                                                                               22.5  27.6                                                                              28.4                                                                              68.3                                                                              57.2                                                                              75.8                                                                              79.8                                                                              36.2                                                                              31.5                         3   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.1 24.0  26.7                                                                              41.1                                                                              56.3                                                                              56.3                                                                              78.3                                                                              87.2                                                                              39.9                                                                              51.5                         4   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.15                                                                               23.7  26.7                                                                              36.3                                                                              49.6                                                                              66.2                                                                              75.6                                                                              86.1                                                                              50.4                                                                              85.1                         5   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.2 25.9  26.7                                                                              43.0                                                                              53.4                                                                              64.0                                                                              71.8                                                                              84.0                                                                              97.4                                                                              120.3                        6   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.25                                                                               23.4  28.6                                                                              50.6                                                                              55.7                                                                              58.2                                                                              74.6                                                                              88.2                                                                              95.5                                                                              106.0                        __________________________________________________________________________                        Azido                                                                              Initial                                                                              Expansion Ratio after Expanding in                Blowing Agent      Silane                                                                             Expansion                                                                            Hot Air at 115° C. for (min.)              Test                                                                               Type   Ratio                                                                              Level                                                                              Level                                                                               Ratio                                                                               (7)                                               No. (1)    (2) (3) (4) (5)   3  5  10 15 30 45 60 90                           __________________________________________________________________________     1   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               --  24.6  29.2                                                                              39.4                                                                              57.2                                                                              65.6                                                                              30.2                                                                              9.6                                                                               7.9                                                                               7.4                          2   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.05                                                                               22.5  35.0                                                                              38.5                                                                              61.8                                                                              57.8                                                                              48.3                                                                              11.6                                                                              8.4                                                                               6.4                          3   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.1 24.0  27.3                                                                              47.7                                                                              66.2                                                                              51.6                                                                              77.7                                                                              16.2                                                                              11.6                                                                              8.4                          4   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.15                                                                               23.7  27.3                                                                              44.1                                                                              58.2                                                                              63.0                                                                              72.6                                                                              13.7                                                                              10.1                                                                              7.6                          5   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.2 25.9  38.2                                                                              42.0                                                                              61.3                                                                              63.0                                                                              91.4                                                                              25.8                                                                              14.3                                                                              11.5                         6   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.25                                                                               23.4  25.8                                                                              40.1                                                                              66.2                                                                              59.2                                                                              83.0                                                                              48.3                                                                              16.8                                                                              11.6                         __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body after expanding in hot air of               110° C. for specified period                                            (7) = expansion ratio of foam body after expanding in hot air of               115° C. for specified period                                      

In the tests a 70/30 mixture of FC-12 and FC-11 was used as the blowing agent. The level of azido silane crosslinking agent was varied up to 0.45 pph. The extruder zones were maintained at about 170°, 200° and 200° C. for feeding, melting, metering, and mixing zone, respectively. The temperature of the cooling zone was adjusted so that the gel could reach an essentially uniform temperature of about 135° C.

When the operating condition reached an essentially steady state, the effects of die opening on foam appearance and line pressure were studied. Foam samples were taken both at the threshold die gap for prefoaming and at a fixed die gap for a given blowing agent system. Property determination and secondary foaming tests were conducted as described in Example I.

In general, the use of the mixed blowing agent of the present invention and silane crosslinking have similar effects on this polymer blend. As shown in Tables IIA, IIB and IIC, azido functional silane increased die pressure and cell size permitting achievement of a larger foam cross section and greater expansion in both steam and hot air. The die may be opened wider at an azido functional silane level 0.25 pph or higher. For this polymer blend also, the type of expanding agent has a pronounced effect on steam expandability. The FC-12/FC-11 mixed blowing agent of the present invention provided good expansion results. This is ascribed to the low permeability and high solubility of FC-11 in polystyrene.

                                      TABLE IIA                                    __________________________________________________________________________                        Azido                                                                              Foaming                                                                             Threshold for Prefoaming                           Blowing Agent      Silane                                                                             Gel  Die                                                                               Die Foam                                                                               Foam                                    Test                                                                               Type   Ratio                                                                              Level                                                                              Level                                                                              Temp.                                                                               Gap                                                                               Press.                                                                             Thick.                                                                             Width                                   No. (1)    (2) (3) (4) (5)  (6)                                                                               (7) (8) (9)                                     __________________________________________________________________________     1   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               --  135  1.803                                                                             43.2                                                                               15.7                                                                               29.7                                    2   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.05                                                                               135  2.032                                                                             42.5                                                                               17.8                                                                               30.5                                    3   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.1 135  2.032                                                                             44.3                                                                               17.3                                                                               32.3                                    4   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.15                                                                               135  2.032                                                                             45.7                                                                               20.0                                                                               34.0                                    5   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.2 135  2.032                                                                             47.1                                                                               20.0                                                                               33.3                                    6   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.25                                                                               135  2.261                                                                             42.9                                                                               20.3                                                                               33.0                                    7   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.35                                                                               135  2.489                                                                             41.5                                                                               28.2                                                                               38.6                                    8   FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.45                                                                               135  2.489                                                                             41.5                                                                               24.6                                                                               33.5                                    __________________________________________________________________________         Sampling                                                                       Die     Die Extruder                                                                              Foam                                                                               Foam  Foam Cell                                     Test                                                                               Gap     Press.                                                                             Press. Thick.                                                                             Width Density                                                                             Size                                     No. (10)    (11)                                                                               (12)   (13)                                                                               (14)  (15) (16)                                     __________________________________________________________________________     1   1.803   43.2                                                                               84.4   15.7                                                                               29.7  33.6 0.30                                         2.032   35.9                                                                               78.0   16.5                                                                               27.4  36.7 --                                       2   2.032   42.5                                                                               91.4   17.8                                                                               30.5  37.5 0.31                                     3   2.032   44.3                                                                               94.2   17.3                                                                               32.3  38.1 0.35                                     4   2.032   45.7                                                                               94.9   20.0                                                                               34.0  37.6 0.38                                     5   2.032   47.1                                                                               96.3   20.0                                                                               33.3  37.5 0.43                                     6   2.032   49.6                                                                               94.9   20.8                                                                               34.3  35.2 0.42                                     7   2.032   54.1                                                                               101.9  26.2                                                                               36.6  35.9 0.56                                     8   2.032   57.0                                                                               103.7  22.4                                                                               35.3  33.0 0.42                                     __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = temperature of foaming gel in degrees centigrade                         (6) = the gap of die opening in millimeters at the threshold of prefoamin      (7) = pressure at the die in kilograms per square centimeter at the            threshold of prefoaming                                                        (8) = thickness of foam body in millimeters at the threshold of prefoamin      (9) = width of foam body in millimeters at the threshold of prefoaming         (10) = the gap of die opening in millimeters where the foam sample was         taken                                                                          (11) = pressure at the die in kilogram per square centimeter at the            sampling die opening                                                           (12) = pressure at the extruder discharge in kilograms per square              centimeter at the sampling die opening                                         (13) = thickness of foam body in millimeters determined after aging for        about a month                                                                  (14) = width of foam body in millimeters determined after aging for about      a month                                                                        (15) = density of foam body in kilograms per square meter determined afte      aging for about a month                                                        (16) = cell size in millimeter in horizontal direction determined per AST      D3576.                                                                   

                                      TABLE IIB                                    __________________________________________________________________________                         Initial                                                                    Azido                                                                              Expan-                                                                             (6)                                                    Blowing Agent   Silane                                                                             sion                                                                               Expansion Ratio after Expanding In Steam for           Test                                                                               Type                                                                               Ratio                                                                              Level                                                                              Level                                                                              Ratio                                                                              15 30 1  3  5  7  10 12 15 20 30451 2                  No. (1) (2) (3) (4) (5) sec.                                                                              sec.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min.                                                                              min. min.                __________________________________________________________________________                                                           hr.hrs.                  1   FC-12/                                                                             70/30                                                                              15.4                                                                               --  25.7                                                                               35.7                                                                              63.2                                                                              80.8                                                                              99.1                                                                              105                                                                               108                                                                               114                                                                               113                                                                               117                                                                               126                                                                               127130116 103                FC-11                                                                      2   FC-12/                                                                             70/30                                                                              15.4                                                                               0.05                                                                               24.6                                                                               33.2                                                                              60.2                                                                              81.4                                                                              101                                                                               110                                                                               109                                                                               121                                                                               112                                                                               129                                                                               136                                                                               138157143 119                FC-11                                                                      3   FC-12/                                                                             70/30                                                                              15.4                                                                               0.1 24.4                                                                               34.3                                                                              62.0                                                                              82.0                                                                              105                                                                               113                                                                               119                                                                               119                                                                               126                                                                               133                                                                               130                                                                               139124131 117                FC-11                                                                      4   FC-12/                                                                             70/30                                                                              15.4                                                                               0.15                                                                               24.6                                                                               30.7                                                                              63.8                                                                              86.2                                                                              114                                                                               127                                                                               131                                                                               142                                                                               141                                                                               145                                                                               160                                                                               166159168 150                FC-11                                                                      5   FC-12/                                                                             70/30                                                                              15.4                                                                               0.2 25.4                                                                               37.1                                                                              71.1                                                                              92.9                                                                              120                                                                               135                                                                               134                                                                               154                                                                               157                                                                               169                                                                               175                                                                               176160170 176                FC-11                                                                      6   FC-12/                                                                             70/30                                                                              15.4                                                                               0.25                                                                               26.7                                                                               38.9                                                                              66.5                                                                              95.8                                                                              128                                                                               143                                                                               152                                                                               157                                                                               167                                                                               198                                                                               187                                                                               190202190 169                FC-11                                                                      7   FC-12/                                                                             70/30                                                                              15.4                                                                               0.35                                                                               26.4                                                                               41.3                                                                              76.4                                                                              103                                                                               144                                                                               164                                                                               178                                                                               201                                                                               202                                                                               202                                                                               203                                                                               220185165 170                FC-11                                                                      8   FC-12/                                                                             70/30                                                                              15.4                                                                               0.45                                                                               27.7                                                                               41.3                                                                              81.7                                                                              99.2                                                                              129                                                                               143                                                                               160                                                                               177                                                                               179                                                                               173                                                                               176                                                                               187199176 167                FC-11                                                                      __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body determined in about two days after          expansion in steam for given period of time                              

                                      TABLE IIC                                    __________________________________________________________________________                       Azido                                                                              Initial                                                                              Expansion Ratio after Expanding in                 Blowing Agent     Silane                                                                             Expansion                                                                            Hot Air at 115° C. for (min.)               Test                                                                              Type   Ratio                                                                              Level                                                                              Level                                                                              Ratio (6)                                                No.                                                                               (1)    (2) (3) (4) (5)   3  5  10 15 30 45 60 90                            __________________________________________________________________________     1  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               --  29.0  56.6                                                                              61.9                                                                              55.7                                                                              38.2                                                                              18.1                                                                               8.6                                                                               7.0                                                                               6.4                          2  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.05                                                                               26.9  54.2                                                                              57.1                                                                              59.2                                                                              56.5                                                                              29.0                                                                              16.4                                                                              10.6                                                                               8.4                          3  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.1 26.3  53.7                                                                              54.1                                                                              62.2                                                                              57.6                                                                              44.0                                                                              17.7                                                                              15.6                                                                              10.0                          4  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.15                                                                               26.5  54.7                                                                              54.4                                                                              61.8                                                                              58.9                                                                              56.8                                                                              51.5                                                                              17.9                                                                              13.2                          5  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.2 27.6  58.0                                                                              57.5                                                                              61.7                                                                              65.2                                                                              69.2                                                                              49.8                                                                              14.6                                                                              13.8                          6  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.25                                                                               28.6  59.2                                                                              61.3                                                                              59.8                                                                              65.0                                                                              72.7                                                                              70.3                                                                              26.3                                                                              15.3                          7  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.35                                                                               28.2  56.1                                                                              58.3                                                                              57.3                                                                              65.3                                                                              69.9                                                                              72.4                                                                              77.1                                                                              42.1                          8  FC-12/FC-11                                                                           70/30                                                                              15.4                                                                               0.45                                                                               29.8  56.2                                                                              65.8                                                                              60.1                                                                              71.2                                                                              70.5                                                                              66.5                                                                              33.7                                                                              16.6                          __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, FC11 = trichloromonofluoromethane        (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body after expanding in hot air of               115° C. for specified period                                      

EXAMPLE III

The apparatus, polymer composition, additives, temperature of extruder zones and operating procedure are the same as in Example II. A 70/30 by weight mixture of FC-12 and isopropyl alcohol was used as the blowing agent.

The process data set forth in Table IIIA manifest the crosslinking-delaying effect of alcohol in the foam extrusion line. The line pressures hardly rise at a silane level as much as 0.20 pph which has resulted in a significant rise in line pressures with FC-12/FC-11 blowing agent (Table IIIA). Only a moderate rise in the line pressures is seen at a higher silane level. With this blowing agent also, the general effects of azido silane on foam extrusion and secondly expansions may be seen.

A close comparison of secondary expansion characteristics between foams expanded with FC-12/FC-11 blowing agent and those expanded with FC-12/i-PrOH blowing agent reveals an interesting tendency. The foams made with FC-12/i-PrOH blowing agent expand faster in steam than those made with FC-12/FC-11 blowing agent but slower than the latter in hot air. It is inferred that isopropyl alcohol not only plasticizes polystyrene but increases the permeation rate of water through the polymer by raising the solubility of water in the polymer/blowing agent phase. For this polymer blend also, the type of expanding agent has a pronounced effect on the steam expandability. The FC-12/i-PrOH mixed blowing agent of the present invention provided very good expansion results.

                                      TABLE IIIA                                   __________________________________________________________________________                        Azido                                                                              Foaming                                                                             Threshold for Prefoaming                           Blowing Agent      Silane                                                                             Gel  Die                                                                               Die Foam                                                                               Foam                                    Test                                                                              Type    Ratio                                                                              Level                                                                              Level                                                                              Temp.                                                                               Gap                                                                               Press.                                                                             Thick.                                                                             Width                                   No.                                                                               (1)     (2) (3) (4) (5)  (6)                                                                               (7) (8) (9)                                     __________________________________________________________________________     1  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               --  134  1.803                                                                             34.5                                                                               16.5                                                                               28.2                                    2  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.05                                                                               134  1.803                                                                             34.1                                                                               18.8                                                                               27.9                                    3  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.1 134  1.803                                                                             34.1                                                                               19.6                                                                               32.3                                    4  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.15                                                                               134  1.803                                                                             33.0                                                                               19.3                                                                               28.4                                    5  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.20                                                                               134  1.803                                                                             32.7                                                                               20.0                                                                               29.7                                    6  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.25                                                                               135  1.803                                                                             35.9                                                                               20.3                                                                               29.7                                    7  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.35                                                                               135  2.032                                                                             29.2                                                                               24.4                                                                               33.5                                    8  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.45                                                                               135  2.286                                                                             31.3                                                                               24.6                                                                               36.3                                    __________________________________________________________________________            Sampling                                                                       Die  Die   Extruder                                                                            Foam  Foam                                                                               Foam   Cell                                          Gap  Press.                                                                               Press.                                                                              Thick.                                                                               Width                                                                              Density                                                                               Size                                   Test No.                                                                              (10) (11)  (12) (13)  (14)                                                                               (15)   (16)                                   __________________________________________________________________________     1      1.803                                                                               34.5  64.0 16.5  28.2                                                                               34.0   0.37                                   2      1.803                                                                               34.1  66.8 18.8  27.9                                                                               30.0   0.39                                   3      1.803                                                                               34.1  66.8 19.6  32.3                                                                               30.4   0.37                                   4      1.803                                                                               33.0  66.8 19.3  28.4                                                                               29.6   0.49                                   5      1.803                                                                               32.7  65.4 20.0  29.7                                                                               28.2   0.46                                   6      1.803                                                                               35.9  73.2 20.3  33.8                                                                               31.9   0.65                                   7      1.803                                                                               29.2  70.3 22.6  28.7                                                                               30.4   0.74                                   8      1.803                                                                               39.7  73.8 21.6  36.8                                                                               31.2   0.52                                   __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, iPrOH = isopropyl alcohol                (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = temperature of foaming gel in degrees centigrade                         (6) = the gap of die opening in millimeters at the threshold of prefoamin      (7) = pressure at the die in kilograms per square centimeter at the            threshold of prefoaming                                                        (8) = thickness of foam body in millimeters at the threshold of prefoamin      (9) = width of foam body in millimeters at the threshold of prefoaming         (10) = the gap of die opening in millimeters where the foam sample was         taken                                                                          (11) = pressure at the die in kilograms per square centimeter at the           sampling die opening                                                           (12) = pressure at the extruder discharge in kilograms per square              centimeter at the sampling die opening                                         (13) = thickness of foam body in millimeters determined after aging for        about a month                                                                  (14) = width of foam body in millimeters determined after aging for about      a month                                                                        (15) = density of foam body in kilograms per square meter determined afte      aging for about a month                                                        (16) = cell size in millimeter in horizontal direction determined per AST      D3576.                                                                   

                                      TABLE IIIB                                   __________________________________________________________________________                        Azido                                                                              Initial                                                                              Expansion Ratio after                             Blowing Agent      Silane                                                                             Expansion                                                                            Expanding In Steam for                            Test                                                                              Type    Ratio                                                                              Level                                                                              Level                                                                              Ratio 15 30 1  3  5                                     No.                                                                               (1)     (2) (3) (4) (5)   sec.                                                                              sec.                                                                              min.                                                                              min.                                                                              min.                                  __________________________________________________________________________     1  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               --  30.1  46.5                                                                              81.9                                                                              98.6                                                                              121                                                                               134                                   2  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.05                                                                               30.6  51.1                                                                              80.8                                                                              102                                                                               129                                                                               137                                   3  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.1 29.0  50.8                                                                              78.8                                                                              102                                                                               123                                                                               133                                   4  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.15                                                                               31.5  58.2                                                                              91.4                                                                              106                                                                               133                                                                               148                                   5  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.2 32.1  51.6                                                                              91.7                                                                              103                                                                               138                                                                               150                                   6  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.25                                                                               29.7  51.0                                                                              92.0                                                                              121                                                                               154                                                                               179                                   7  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.35                                                                               31.1  56.0                                                                              96.7                                                                              117                                                                               166                                                                               149                                   8  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.45                                                                               31.3  61.2                                                                              99.4                                                                              126                                                                               174                                                                               195                                   __________________________________________________________________________     Expansion Ratio after Expanding In Steam for                                   Test                                                                              7                                  1  2                                     No.                                                                               min. 10 min.                                                                            12 min.                                                                              15 min.                                                                            20 min.                                                                              30 min.                                                                            45 min.                                                                              hr.                                                                               hrs                                   __________________________________________________________________________     1  147  137 144   143 144   143 125   126                                                                               112                                   2  146  155 153   160 154   156 153   147                                                                               131                                   3  152  155 157   174 161   166 137   150                                                                               150                                   4  156  137 164   160 169   166 162   167                                                                               151                                   5  185  220 171   172 194   163 158   175                                                                               148                                   6  197  209 174   192 190   201 170   122                                                                               134                                   7  198  207 203   209 207   197 168   178                                                                               131                                   8  202  198 191   198 202   185 135   158                                                                               131                                   __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, iPrOH = isopropyl alcohol                (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body determined in about two days after          expansion in steam for given period of time                              

                                      TABLE IIIC                                   __________________________________________________________________________                        Azido                                                                              Initial                                                 Blowing Agent      Silane                                                                             Expansion                                               Test                                                                              Type    Ratio                                                                              Level                                                                              Level                                                                              Ratio                                                   No.                                                                               (1)     (2) (3) (4) (5)   3  5  10 15 30 45 60 90                           __________________________________________________________________________                                  Expansion Ratio after Expanding in                                             Hot Air at 115° C. for (min.)                                           (6)                                               1  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               --  30.1  34.7                                                                              44.1                                                                              45.8                                                                              53.6                                                                              56.7                                                                              58.8                                                                              64.1                                                                              15.8                         2  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.05                                                                               32.0  34.7                                                                              34.4                                                                              54.4                                                                              51.6                                                                              59.2                                                                              64.0                                                                              67.8                                                                              26.7                         3  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.1 30.9  31.5                                                                              46.4                                                                              52.5                                                                              52.5                                                                              56.9                                                                              62.1                                                                              65.6                                                                              60.1                         4  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.15                                                                               33.1  33.1                                                                              45.8                                                                              54.3                                                                              52.2                                                                              59.5                                                                              64.8                                                                              67.4                                                                              49.9                         5  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.2 34.1  43.8                                                                              58.2                                                                              54.3                                                                              61.1                                                                              62.1                                                                              64.0                                                                              66.5                                                                              66.8                         6  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.25                                                                               29.6  35.7                                                                              47.9                                                                              47.3                                                                              51.5                                                                              51.2                                                                              52.5                                                                              57.4                                                                              74.2                         7  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.35                                                                               30.3  30.2                                                                              48.2                                                                              50.5                                                                              50.0                                                                              51.9                                                                              56.4                                                                              59.9                                                                              74.1                         8  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.45                                                                               31.0  34.0                                                                              39.7                                                                              55.8                                                                              64.5                                                                              65.8                                                                              65.8                                                                              65.0                                                                              81.9                                                      Expansion Ratio after Expanding in                                             Hot Air at 120° C. for (min.)                                           (7)                                               1  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               --  30.1  31.1                                                                              33.2                                                                              47.7                                                                              47.7                                                                              11.5                                                                               9.6                                                                               8.4                                                                               7.0                         2  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.05                                                                               32.0  32.5                                                                              40.1                                                                              56.3                                                                              56.7                                                                              23.1                                                                              12.6                                                                               8.4                                                                               8.1                         3  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.1 30.9  36.8                                                                              30.6                                                                              51.6                                                                              56.9                                                                              24.8                                                                              17.2                                                                              13.4                                                                              11.4                         4  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.15                                                                               33.1  33.3                                                                              40.1                                                                              56.9                                                                              58.8                                                                              40.1                                                                              16.2                                                                              14.3                                                                              11.5                         5  FC-12/i-PrOH                                                                           70/30                                                                              15.0                                                                               0.2 34.1  47.7                                                                              46.8                                                                              59.2                                                                              57.3                                                                              40.1                                                                              16.2                                                                              13.4                                                                              10.5                         6  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.25                                                                               29.6  33.1                                                                              40.4                                                                              56.9                                                                              61.4                                                                              58.6                                                                              25.4                                                                              18.4                                                                              13.1                         7  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.35                                                                               30.3  38.8                                                                              44.4                                                                              61.4                                                                              59.3                                                                              73.5                                                                              31.5                                                                              20.3                                                                              13.7                         8  FC-12/i-PrOH                                                                           70/30                                                                              15.1                                                                               0.45                                                                               31.0  35.0                                                                              50.1                                                                              64.8                                                                              66.5                                                                              80.5                                                                              66.8                                                                              31.5                                                                              21.0                         __________________________________________________________________________      (1) = FC12 = dichlorodifluoromethane, iPrOH = isopropyl alcohol                (2) = mixture ratio by weight of the component blowing agents                  (3) = parts of blowing agent mixed in per hundred parts of polymer             (4) = parts of azido functional silane mixed in per hundred parts of           polymer                                                                        (5) = specific volume of foam body aged for about a month divided by           specific volume of the polymer determined prior to steam expansion tests       (6) = expansion ratio of foam body after expanding in hot air of               115° C. for specified period                                            (7) = expansion ratio of foam body after expanding in hot air of               120° C. for specified period                                      

EXAMPLE IV

The apparatus and procedures used in this example were essentially the same as those used in previous examples. A polystyrene with a molecular weight of 200,000 was used. A steam chamber such as that illustrated in FIG. 1 was attached to the outlet die of the extruder to determine the effect of on-line steam application on foam density. Three steam chambers of 0.5, 1.0, and 2.0 feet (15.2, 25.4, and 50.8 cm) in length were used to provide different residence times. The steam chambers consisted of a Lexan® polycarbonate tube having a nominal 2.875 inches (7.3 cm) I.D. and a nominal 3.0 inches (7.6 cm) O.D., an outer shielding tube made of 0.25 inch (0.635 cm) thick, 4 inch (10.16 cm) O.D. Plexiglass®, a steam injection assembly attached at the die face, and the end piece having a rubber seal. The steam injection assembly and the end piece were commonly used for all three tubes. Steam was generated by a small boiler and was flowed into the chamber through a Tegraglass® porous distributor. The steam impinged on the surface of the expanding foam strand at the die, passed through the chamber, and was permitted to leak out through the clearance between the foam strand and rubber seal. The foam strand was conducted away at a uniform rate by a set of pull rolls.

The pull rate was adjusted to give a slight pull to the strand. Control samples were taken prior to steam tests with each chamber. The steam pressure at the boiler was maintained at about 20 psig (1.41 kg/cm²). After steam was introduced, a few minutes were allowed for the system to equilibrate prior to taking samples.

FIG. 3 illustrates how different blowing agent formulations affect the steam expansion rates of foams extruded from a polystyrene having a weight average molecular weight of about 200,000. Since factors affecting the apparent expansion rate include foam dimensions, foam densities, and foaming temperature, corrections were made for these external factors so that intrinsic expansion rates would be properly compared. Corrections for dimensional variations were made according to the following equation: ##EQU1## where T* is the corrected exposure time and T is the real exposure time. The normalized corrected exposure time (T*) is approximately equivalent to the exposure time for a cylindrical foam having a one inch diameter. No corrections were made for the variation of foam density since they were all within a narrow range of 1.9 to 2.3 pcf (30.4 to 36.8 Kg/m³).

As can be seen from FIG. 3, the foams made with blowing agents within the scope of the present invention expanded rapidly. The addition of azido silane crosslinking agent further enhances the steam expansion rate. For this foam, density was reduced by 10% in 4 seconds and 20% in about 6 seconds of steam exposure.

FIG. 4 shows a similar effect for a mixed FC-12/i-PrOH blowing agent within the scope of the present invention on a 50/50 blend of polystyrenes having weight average molecular weights of approximately 200,000 and 300,000, respectively.

Table IV below summarizes all data for on-line steam expansion in terms of steam exposure time for a 10% reduction in foam density. The data have been normalized for a 1.0 inch diameter, 2.0 pcf polystyrene foam.

                  TABLE IV                                                         ______________________________________                                                                          Time (sec.) for                               Polymer               Azido Silane                                                                              10% Reduction                                 Type   Blowing Agent  Level (pph)                                                                               in Density                                    ______________________________________                                         A      FC-12/EtOH:70/30                                                                              --         4.6                                           A      FC-12/EtOH:70/30                                                                              0.5        3.9                                           B      FC-12/EtOH:70/30                                                                              --         6.9                                           B      FC-12/EtOH:70/30                                                                              0.2        4.8                                           B      FC-12/EtOH:70/30                                                                              0.5        3.8                                           B      FC-12/i-PrOH:70/30                                                                            --         8.2                                           ______________________________________                                          A = polystyrene having Mw = 200,000                                            B = 50/50 blend of polystyrenes having Mw = 200,000 and 300,000,               respectively                                                             

As can be seen, foams made with blowing agents within the scope of the present invention expand rapidly.

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. 

What is claimed is:
 1. A process for the preparation of an expanded polymeric foam comprising the steps of:(a) melt processing under pressure an alkenyl aromatic polymer having a glass transition temperature of between about 60° to about 100° C. together with a volatile blowing agent to form a flowable admixture, said volatile blowing agent comprising a mixture of from about 40 to about 90% by weight of a primary blowing agent comprising dichlorodifluoromethane, and from about 10 to about 60% by weight of a secondary blowing agent comprising a halogenated hydrocarbon, a hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms, said halogenated hydrocarbon, hydrocarbon or aliphatic alcohol having a normal atmospheric boiling point of from about 0° to about 100° C., (b) extruding said admixture through a die to a zone of lower pressure and activating said blowing agent to expand said alkenyl aromatic polymer to a cellular structure, and (c) exposing said cellular structure to steam at substantially atmospheric pressure to expand further said cellular structure to a foam of lower density.
 2. The process of claim 1 in which said alkenyl aromatic polymer is polystyrene.
 3. The process of claim 1 in which said alkenyl aromatic polymer is a copolymer of styrene and acrylonitrile.
 4. The process of claim 1 in which said volatile blowing agent is present in an amount of from 10-30 parts per hundred of said alkenyl aromatic polymer.
 5. The process of claim 1 in which said volatile blowing agent comprises from about 60 to about 80% by weight dichlorodifluoromethane and from about 20 to about 40% by weight trichloromonofluoromethane.
 6. The process of claim 1 in which an azido functional silane crosslinking agent is added in step (a).
 7. A process for the preparation of an expanded polymeric foam comprising the steps of:(a) impregnating a volatile blowing agent into a granular alkenyl aromatic polymer having a glass transition temperature of between about 60° to about 100° C., said volatile blowing agent comprising a mixture of from about 40 to about 90% by weight of a primary blowing agent comprising dichlorodifluoromethane and from about 10 to about 60% by weight of a secondary blowing agent comprising a halogenated hydrocarbon, hydrocarbon, or an aliphatic alcohol containing from 1 to 4 carbon atoms, said halogenated hydrocarbon, hydrocarbon, or aliphatic alcohol having a normal atmospheric boiling point of from about 0° to about 100° C., (b) heating said granular alkenyl aromatic polymer to activate said volatile blowing agent and expand said granular alkenyl aromatic polymer to a cellular structure, and (c) exposing said cellular structure to steam at substantially atmospheric pressure to expand further said cellular structure to a foam of lower density.
 8. The process of claim 7 in which steps (b) and (c) occur substantially simultaneously and said steam supplies the necessary heat to activate said volatile blowing agent.
 9. The process of claim 7 in which said alkenyl aromatic polymer is polystyrene.
 10. The process of claim 7 in which said alkenyl aromatic polymer is a copolymer of styrene and acrylonitrile.
 11. A process of claim 7 in which said volatile blowing agent is present in an amount of from 10-30 parts per hundred of said alkenyl aromatic polymer.
 12. The process of claim 7 in which said volatile blowing agent comprises from about 60 to about 80% by weight dichlorodifluoromethane and from about 20 to about 40% by weight trichloromonofluoromethane.
 13. The process of claim 7 in which an azido functional silane crosslinking agent is added to said alkenyl aromatic polymer in step (a). 